1. Technical Field
The present invention relates to a communication apparatus and an SRS transmission control method.
2. Description of the Related Art
3rd Generation Partnership Project Radio Access Network Long Term Evolution (3GPP-LTE) (hereinafter, referred to as “LTE”) employs Orthogonal Frequency Division Multiple Access (OFDMA) as the downlink communication scheme and employs Single Carrier Frequency Division Multiple Access (SC-FDMA) as the uplink communication scheme (see, Non-Patent Literatures (hereinafter, abbreviated as NPL) 1, 2 and 3, for example). In addition, a Periodic Sounding Reference signal (P-SRS) is used in the uplink of LTE as a reference signal for measuring uplink reception quality.
In order for terminals to transmit the P-SRS to a base station, an SRS transmission subframe shared by all terminals (hereinafter, referred to as “common SRS subframe”) is configured. This common SRS subframe is defined by a combination of a predetermined periodicity and a subframe offset on a per cell basis. The information about the common SRS subframe is broadcasted to the terminals in the cell. For example, when the periodicity is 10 subframes, and the offset is 3, a third subframe in a frame (formed of 10 subframes) is configured as a common SRS subframe. In the common SRS subframe, all the terminals in the cell stop the transmission of data signals at the last SC-FDMA symbol of the subframe and use this period as a transmission resource for reference signals.
In addition, each terminal is specifically configured with an SRS transmission subframe by a higher layer (RRC layer above the physical layer) (hereinafter, referred to as “specific SRS subframe”). Each terminal transmits a P-SRS in the configured specific SRS subframe. In addition, each terminal is configured with parameters on the SRS resource (hereinafter, may be referred to as “SRS resource parameter”) and also notified of the parameter. The parameters on the SRS resource include the bandwidth and band position of the SRS (or position where SRS band starts), Cyclic Shift, Comb (which corresponds to identification information on the subcarrier group), and/or the like. The terminal transmits an SRS using the resource in accordance with the notified parameters. In addition, SRS frequency hopping may be configured in some cases.
In addition, the introduction of a dynamic aperiodic SRS (hereinafter, referred to as “A-SRS”) into the uplink of LTE-Advanced, which is an advanced version of LTE (hereinafter, referred to as “LTE-A”), has been discussed. The transmission timing of an A-SRS is controlled by trigger information (e.g., 1-bit information). This trigger information is transmitted to a terminal from a base station on a physical layer control channel (i.e., PDCCH) (e.g., see NPL 4). More specifically, the terminal transmits an A-SRS only in response to an A-SRS transmission request made by the trigger information (i.e., A-SRS transmission request). In addition, studies have been carried out on defining, as the A-SRS transmission timing, the first common SRS subframe located at or after a k-th subframe (e.g., k=4) from the subframe in which the trigger information is transmitted. As described above, although a P-SRS is transmitted periodically, it is possible to cause a terminal to transmit an A-SRS frequently within a short period only during a data burst in the uplink transmission, for example.
Moreover, LTE-A provides control information formats for various types of data assignment reporting. The control information formats in the downlink include: DCI format 1A for allocation of resource blocks consecutive in number (Virtual RBs or Physical RBs); DCI format 1, which allows allocation of RBs not consecutive in number (hereinafter, referred to as “non-contiguous bandwidth allocation”); DCI formats 2 and 2A for assigning a spatial-multiplexing MIMO transmission; a downlink assignment control information format for assigning a beam-forming transmission (“beam-forming assignment downlink format:” DCI format 1B); and a downlink assignment control information format for assigning a multi-user MIMO transmission (“multi-user MIMO assignment downlink format:” DCI format 1D), and/or the like. Meanwhile, the uplink assignment formats include DCI format 0 for assigning a single antenna port transmission and DCI format 4 for assigning an uplink spatial-multiplexing MIMO transmission. DCI format 4 is used for only terminals configured with an uplink spatial-multiplexing MIMO transmission.
In addition, DCI format 0 and DCI format 1A are adjusted in size by padding so that each format consists of the same number of bits. DCI format 0 and DCI format 1A are also called DCI format 0/1A in some cases. DCI formats 1, 2, 2A, 1B and 1D are used depending on the downlink transmission mode configured for each terminal (i.e., non-contiguous bandwidth allocation, spatial-multiplexing MIMO transmission, beam-forming transmission or multi-user MIMO transmission) and are configured for each terminal. Meanwhile, DCI format 0/1A can be used independently of the transmission mode and thus can be used for terminals in any transmission mode, i.e., DCI format 0/1A is a format commonly usable in all terminals. In addition, when DCI format 0/1A is used, single-antenna transmission or transmit diversity is used as the default transmission mode.
A terminal receives DCI format 0/1A, and the DCI formats that are dependent on the downlink transmission mode. In addition, a terminal configured with an uplink spatial-multiplexing MIMO transmission receives DCI format 4 in addition to the abovementioned DCI formats.
The use of DCI format 0 and DCI format 4, which are control information formats used for uplink data (PUSCH) assignment reporting, for reporting the A-SRS trigger information has been discussed. The field for reporting an A-SRS trigger is added to DCI format 0 in addition to an RB reporting field, MCS reporting field, HARQ information reporting field, transmission power control command reporting field and terminal ID field. In addition to the fields described above, DCI format 4 includes an MCS reporting field for the second transport block (data codeword) to be spatially multiplexed, and precoding information for spatial multiplexing.
The DCI described above is transmitted to a base station to a terminal via a PDCCH. The base station in this case assigns a plurality of terminals to a single subframe, so that the base station simultaneously transmits a plurality of PDCCHs using different resources. The base station transmits the PDCCHs while including CRC bits which have been masked (or scrambled) using the terminal ID of the transmission destination in each of the PDCCHs in order to identify the terminal of the transmission destination of each of the PDCCHs. Each terminal then detects the PDCCH intended for the terminal by blind-decoding the PDCCHs by demasking (or descrambling) CRC bits with the terminal ID of the terminal in the PDCCHs which may have been transmitted for the terminal.